The present invention relates to improvements in a system for detecting the position of a surface of a workpiece such as a wafer with excellent accuracy without touching the workpiece.
For example, in an electron beam exposure system in which a wafer is exposed directly to an electron beam without having any mask on the wafer, it is required to measure the height of the exposed surface of the wafer in order to place the focal point of the electron beam on the exposed surface of the wafer. When the electron beam has a depth of focus of about 5 .mu.m, it is desirable to control the height of the exposed surface within about 5 .mu.m. On the other hand, the height of the exposed surface is usually changed by about 500 .mu.m. Therefore, the height of the exposed surface is measured with infrared rays, to which an electron beam resist material is insensitive. Since the exposed surface of the wafer may have been provided with various patterns (oxide, electrode, etc.), the reflectivity of the exposed surface for the infrared rays varies with the position, or there may be such surface part in which a plurality of portions different in reflectivity from each other are mixed together. In the case where the height of such an exposed surface is measured each time the wafer is moved, a serious error arises in a measured value due to a change in reflectivity or distribution of reflectivity at a measured region.
There are many methods for measuring the position (or altitude) of a surface of a workpiece. However, there are only a limited number of methods in which a measuring device can be mounted in an evacuated chamber having a limited volume as in an electron beam exposure machine and in which measurement can be done without causing interference with other devices and without exposing the resist film applied on the surface. For example, a method of measuring the height of a wafer is proposed in which a wafer is irradiated with a divergent light beam and the height of the wafer is determined on the basis of the intensity of reflected light incident on a predetermined light-receiving element. Another method is proposed in which a light beam is focused on a wafer to form a light spot, an image of the light spot is formed on a detector by the reflected light, and the height of the wafer is determined by the position of the image on the detector. An example of the latter method is disclosed in Japanese Patent Application No. 133,076/1975. In the case where a surface of a wafer is provided with a pattern and therefore the reflectivity of the surface varies with the position, the intensity of light which is reflected from the surface of the wafer and then incident on a detector, and the intensity distribution of the above light vary depending upon the positions at which measuring light is incident on the surface of the wafer. In this case, it is impossible to use the former method. Further, there arises in the latter method problems that the accuracy with which the position of the wafer is measured, is decreased and therefore it is very difficult to obtain a correct measured value.